1. Field of the Invention
The present invention relates to an oxidation catalyst useful for chemical synthesis. More particularly, the present invention is concerned with an oxidation catalyst for use in the oxidation of a substrate with a molecular oxygen, comprising at least one member selected from the group consisting of a specific hydrazyl radical and a specific hydrazine compound. By the use of the oxidation catalyst of the present invention, oxidation of a substrate (such as a hydrocarbon, an alcohol, a carbonyl compound, an ether, an amine, a sulfur compound or a heterocyclic compound) with a molecular oxygen can be efficiently performed under moderate conditions, thereby enabling economical production of a useful chemical compound with high selectivity. The present invention is also concerned with a method for producing a chemical compound by using the oxidation catalyst of the present invention.
2. Prior Art
An oxidation reaction is one of the most basic methods for material conversion in the field of organic synthesis. A number of oxidation processes have been put into practical use. However, in many cases of oxidation reactions, for example, oxygen oxidation reactions, there are disadvantages not only in that high temperature and high pressure conditions are necessary for activation of a molecular oxygen, thus leading to a lowering of the selectivity for a desired compound, but also in that, because of the generation of various by-products, it is necessary to perform various separation steps for removing the by-products. Therefore, the conventional oxidation processes are not at a satisfactory technical level from the viewpoint of economy, safety, and environmental protection. Thus, it has been desired to develop a new oxidation process which can be efficiently performed with high selectivity and at a low cost.
Conventionally, oxidation reactions which are well known in the industry, especially a selective oxidation reaction, have been carried out by employing the techniques as described below.
Useful chemical compounds can be efficiently produced under moderate conditions with high selectivity by performing a selective oxidation reaction, using an oxidizing agent (such as hydrogen peroxide or an organic or inorganic peroxide) which can produce an active oxygen species (an electrophilic oxygen species) having high chemical potential (see, for example, “Shin Jikken Kagaku Koza 15, Sanka to Kangen I-2 (New Lecture on Experimental Chemistry 15, Oxidation and Reduction, I-2)”, edited by the Japan Chemical Society, p. 605, 1976, Japan; and “Catalytic Oxidations with Hydrogen Peroxide as Oxidant”, G. Strukul, Kluwer Academic Publishers, 1992, the Netherlands). Examples of selective oxidation reactions include oxidation of an alkane, oxidation of an alcohol, epoxidation of an olefin, oxidation of a ketone, oxidation of an aldehyde, oxidation of an ether, hydroxylation of an aromatic compound, oxidation of an amine and oxidation of a sulfur compound.
On the other hand, it is well known that, as a conventional method for producing hydrogen peroxide, which is a useful oxidizing agent as mentioned above, an autoxidation reaction using alkylanthraquinone is commercially used (see “Kagaku Binran, Oyo-kagaku-hen I (Chemical Handbook, Applied Chemistry I)”, edited by the Japan Chemical Society, p. 302, 1986, Japan). However, the conventional method for producing hydrogen peroxide is economically disadvantageous not only in that the method requires a large amount of an organic solvent, but also in that, due to the generation of various by-products and degradation of a catalyst, the method requires various additional steps for separation of by-products and for regeneration of the degraded catalyst. Therefore, it has been desired to develop a production method by which hydrogen peroxide can be produced at a low cost, as compared to the case of the conventional method.
In addition, as a useful organic peroxide, t-butylhydroperoxide is also known. Conventionally, t-butylhydroperoxide has been produced, for example, by a method in which t-butanol or isobutylene as a substrate, namely a raw material, is reacted with a strong acid, such as sulfuric acid, and hydrogen peroxide (see, for example, “Yuki-Kasankabutsu (Organic Peroxides)”, edited by the Organic Peroxide Research Group, p. 220, 1972, Japan). However, the conventional method is disadvantageous from the viewpoint of economy and safety; specifically, the conventional method has disadvantages in that hydrogen peroxide (which is expensive) is necessary, and the raw material is reacted with a liquid mixture of a high concentration aqueous sulfuric acid (60 to 70 wt %) and a high concentration aqueous hydrogen peroxide (30 to 50 wt %).
For these reasons, from the practical and commercial viewpoint, it has been desired to develop a method by which various types of selective oxidation reactions mentioned above can be performed by directly activating and oxidizing a substrate with oxygen in the presence of a catalyst without using an expensive oxidizing agent, such as hydrogen peroxide. For example, a method for producing phenol directly from benzene and oxygen in the presence of a catalyst has long been studied. However, the reaction method which has been studied has the following problems. First, a high temperature is necessary for the reaction. Further, although various types of catalysts can catalyze the reaction, many of such catalysts pose a problem in that the reaction system containing such catalysts causes phenol as a reaction product to have higher reactivity than benzene as a substrate, so that, although the reaction rate of benzene can be increased, the selectivity for phenol is decreased. Thus, the above-mentioned method is not commercially employable. With respect not only to such reaction system (which causes phenol as a reaction product to have higher reactivity than benzene) but also to other oxidation reactions using oxygen, great efforts have been made for increasing the selectivity for a desired reaction product. However, there is no method which is satisfactory from the viewpoint of economy and safety. It is considered that the reason why such an oxidation reaction using oxygen does not proceed with high selectivity for a desired product resides in that, when oxygen molecules are activated by a catalyst, an electron transfer from the catalyst to the oxygen molecules inevitably occurs, so that oxygen molecules are mainly converted to nucleophilic oxygen anion active species, thus rendering it difficult for an electrophilic addition reaction to proceed (see Catalysis Today, 45, 3-12, 1998, the U.S.A.).
In recent years, in order to alleviate the above-mentioned problems, studies on a new method have been made for synthesizing a chemical compound, in which a catalyst system which is similar to a biological catalyst system is used. Monooxygenase, which is an enzyme present in the living body, activates an oxygen molecule by utilizing the reducing ability of NADPH. In imitation of this mechanism, in the synthetic chemistry, a method can be used in which oxygen and a reducing agent, such as hydrogen, carbon monoxide, aldehyde or hydrazine, are contacted with each other in the presence of a catalyst system, thereby generating an active oxygen species stoichiometrically under moderate conditions. In this case, since energy necessary to cleave an oxygen bond is supplied through the oxidation of the reducing agent, an electrophilic active oxygen species can be selectively generated without using a large amount of energy.
There are also known methods similar to such method, such as a method for producing phenol, comprising contacting oxygen, benzene and hydrogen with each other in the presence of a Pt—V2O5/SiO2 catalyst (Appl. Catal., A, 131, 33, 1995, U.S.A.); a method for producing cyclohexene oxide, comprising contacting oxygen, cyclohexene and hydrogen with each other in the presence of an Mn complex/Pt colloidal catalyst (J. Am. Chem. Soc., 101, 6456, 1979, U.S.A.); a method for producing cyclohexanone and cyclohexanol, comprising contacting oxygen, cyclohexane and acetoaldehyde with each other in the presence of an Fe2O3 catalyst in an acetic acid solvent (J. Mol. Catal., A: Chemical, 117, 21, 1979); and a method for producing cyclohexanone and cyclohexanol, comprising contacting oxygen, cyclohexane and 1,2-diphenylhydrazine with each other in the presence of an Fe(pyridine)4Cl2 catalyst (Polyhedron, 7(6), 425, 1988). These methods exhibit an improved selectivity for a desired product. However, each of these methods has problems not only in that the presence of a reducing agent poses a high danger of explosion, but also in that the use efficiency of a reducing agent is low and an oxidation product of a reducing agent is by-produced. Therefore, these methods cannot be commercially practically employed.
On the other hand, as described below, studies are now being performed for developing a new oxidation catalyst which can be used to perform oxidation of various substrates with a molecular oxygen under moderate conditions without using a reducing agent.
There has been proposed a method in which a substrate, such as an alkane, an alcohol or a ketone, is subjected to oxidation with a molecular oxygen by using, as an oxidation catalyst, an imido compound, such as N-hydroxyphthalimide (see, for example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-286467 (corresponding to U.S. Pat. No. 5,981,420 and EP 858835 B1)). In this method, selective oxidation of a substrate can be performed under moderate conditions. However, this method poses problems not only in that the catalyst activity of an imido compound is unsatisfactory, thus rendering it necessary to use the imido compound catalyst in an amount as large as about 10 mole %, based on the molar amount of the substrate, but also in that the imido compound is decomposed and consumed during the reaction, thus increasing the cost of producing a chemical compound by this method.
Also, it is known to use as a catalyst a nitroxyl radical (such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)) in a method for selective oxidation of an alcohol (see Chem. Comm., p. 1591, 1999). In this method, an alcohol is converted selectively into a carbonyl compound or the like by using a catalyst comprising a combination of TEMPO and a ruthenium compound. However, this method poses problems in that not only is the catalyst activity unsatisfactory, but also the nitroxyl radical undergoes degradation during the reaction.
As methods in which a metal metalloporphyrin complex or a metallosalen complex is used as a catalyst, there can mentioned, for example, an oxidative acetoxylation reaction of a diene by using a ternary catalyst system of [Pd salt/Co-porphyrin complex/hydroquinone] (see Angew. Chem. Int. Ed. Engl., 32, 263, 1993), a method in which a ternary catalyst system of [Ru complex/Co-salen complex/hydroquinone] is used to perform an oxidation reaction of a primary alcohol to form aldehyde or an oxidation reaction of a secondary alcohol to form a ketone (see J. Chem. Soc., Chem. Commun., p. 1037, 1994), an oxidation reaction of an amine by using an azaindole/copper complex (see “Nihon Kagakukai Dai-67 Shunki Nenkai Kouen Yokou-shu II (the preliminary text II for the lectures at the 67th Spring Annual Meeting of the Chemical Society of Japan), p. 1025, 1994, Japan), an oxidative dehydrogenation reaction of an amine by using a heteropolyacid (see ° Nihon Kagakukai Dai-67 Shunki Nenkai Kouen Yokou-shu II (see the preliminary text II for the lectures at the 67th Spring Annual Meeting of the Chemical Society of Japan), p. 761, 1994, Japan), and an oxidation reaction of an alkane by using iron-disubstituted tungstosilicic acid (Chem. Lett., p. 1263, 1998). However, these methods pose problems not only in that the reaction rate and the selectivity for and yield of a desired chemical compound are not always high, but also in that the catalyst has the following disadvantages: the preparation of the catalyst requires a complicated operation and, hence, the catalyst is expensive, and also the catalyst is unstable and likely to be decomposed during the reaction.
Next, an explanation is made below on the conventional methods for producing an oxime compound, a nitro compound and a nitrone compound which are induced by oxidation of an amine.
Oxime compounds and nitro compounds are important chemical compounds for use as, for example, ordinary and fine chemicals and intermediates for synthesis of pharmaceuticals. Examples of methods for producing an oxime compound by oxidation of a primary amine include a method which uses dimethyl dioxirane as an oxidizing agent (see J. Org. Chem., 57, 6759, 1992), a method which uses hydrogen peroxide as an oxidizing agent and uses a sodium tungsten catalyst (see Angew. Chem., 72, 135, 1960), a method which uses hydrogen peroxide as an oxidizing agent and uses a methyltrioxorhenium catalyst (see Bull. Chem. Soc. Jpn., 70, 877, 1997), and a method which uses hydrogen peroxide as an oxidizing agent and uses a titanium silicalite molecular sieves (TS-1) as a catalyst (see J. Chem. Soc. Perkin Trance. I, 2665, 1993). However, these methods have a problem in that an explosive oxidizing agent (dimethyl dioxirane or hydrogen peroxide) is used or that an expensive oxidizing agent or an expensive catalyst is used. These methods are also disadvantageous in that the catalyst activity is unsatisfactory and the selectivity for and yield of an oxime compound are not always high. Therefore, these methods are commercially unsatisfactory.
Examples of methods for producing a nitro compound by oxidation of a primary amine include a method which uses m-chloroperbenzoic acid or a pertrifluoroacetic acid as an oxidizing agent (see J. Org. Chem., 58, 1372, 1993), a method which uses potassium permanganate as an oxidizing agent (see Org. Synth., 52, 77, 1972), a method which uses ozone as an oxidizing agent (see Synthetic Commun., 20, 1073, 1990), a method which uses t-butylhydroperoxide as an oxidizing agent and uses a catalyst comprising chromium carried on a silica carrier (see J. Chem. Soc. Commun., 1523, 1995), and a method which uses dimethyl dioxirane as an oxidizing agent (see Tetrahedron Lett., 27, 2335, 1986). However, these methods have a problem in that an expensive peroxide, an excess amount of a heavy metal-containing oxidizing agent or an explosive oxidizing agent (such as ozone, an organic hydroperoxide or dimethyl dioxirane) is used. Therefore, these methods are commercially unsatisfactory.
Cyclohexanone oxime, which is an oxime compound, is a chemical compound useful as an intermediate for producing ε-caprolactam which is a raw material for producing nylon-6. Cyclohexanone oxime can be synthesized by reacting a cyclohexyl amine as a raw material with an oxidizing agent. Examples of methods for producing cyclohexanone oxime by using hydrogen peroxide as an oxidizing agent include (1) a method which uses a catalyst containing at least one metal selected from the group consisting of Mo, W and U (see U.S. Pat. No. 2,706,204), (2) a method which uses titanium silicalite or vanadium silicalite as a catalyst (see Tetrahedron, 51(41), 11305, 1995, and Catal. Lett., 28(2 to 4), 263, 1994), and (3) a method which uses a catalyst containing at least one metal selected from the group consisting of Ti, V, Cr, Se, Zr, Nb, Mo, Te, Ta, W, Re and U (see U.S. Pat. No. 3,960,954).
Examples of methods for producing cyclohexanone oxime by using a molecular oxygen as an oxidizing agent include (4) a method in which a gaseous phase reaction is performed using a solid catalyst comprising an SiO2 gel, γ-Al2O3 and optionally WO3 (see U.S. Pat. Nos. 4,337,358 and 4,504,681), (5) a method in which a gaseous phase reaction is performed using a solid catalyst comprising a combination of tungsten oxide and one member selected from the group consisting of γ-Al2O3, SiO2 and hydrotalcite (see Journal of Molecular Catalysis A; Chemical, 160, 393, 2000), (6) a method in which a liquid phase reaction is performed using, as a catalyst, tungstic acid, phosphotungstic acid, molybdic acid, selenic acid; selenious acid or the like in the presence of a tertiary alcohol, preferably in the presence of a tertiary alcohol and ammonia gas (see Examined Japanese Patent Application Publication No. Sho 47-25324), and (7) a method in which a liquid phase reaction is performed using, as a catalyst, a compound containing at least one element selected from the Group 4 (Ti, Zr and Hf) of the Periodic Table (see EP 395046 B1).
However, these prior art methods have problems. For example, the above-mentioned methods (1) to (3) have problems not only in that the oxidizing agent used (i.e., hydrogen peroxide or an organic hydroperoxide) is expensive, but also in that the selectivity for and yield of the cyclohexanone oxime produced are not always high, and also there is the known operational danger (explosiveness) of the oxidizing agent when the reaction is performed on a commercial scale. In addition, when an organic hydroperoxide is used, there is also a problem in that a by-product generated by reduction of the organic hydroperoxide is contained in the reaction mixture, thus rendering cumbersome the separation and purification operations. For solving these problems, the above-mentioned methods (4) to (7), which use a molecular oxygen (such as air or oxygen gas), have been proposed.
In the above-mentioned methods (4) and (5), the gaseous phase reaction is performed under relatively stringent reaction conditions using a reaction temperature of from 120 to 250° C. In the studies by the present inventors, it has been found that the above-mentioned methods (4) and (5) pose a problem in that, when the reaction temperature is 160° C. or more, a tar-like by-product and a high boiling point organic carbonaceous material accumulate on the surface of the catalyst, thus causing a rapid deactivation of the catalyst. Also, the methods (4) and (5) are disadvantageous in that the selectivity for cyclohexanone oxime is as low as about 50 to 60% at a conversion of 20%, thus causing a lowering of the amount of reaction product per unit volume of the space of reaction, i.e., a lowering of the productivity. In the above-mentioned methods (6) and (7), the gaseous phase reaction is performed under relatively moderate reaction conditions using a reaction temperature of from 50 to 150° C. The prior art document describing the method (6) discloses a reaction in which t-butanol is used as a reaction solvent and phosphotungstic acid is used as a catalyst; however, the method (6) has a problem in that the yield of cyclohexanone oxime is as low as only few percent. The prior art document describing the method (7) discloses a reaction in which a titanium compound is used as a catalyst, and diethylene glycol dimethyl ether (i.e., diglyme), t-butanol, dimethyl formamide, acetonitrile, triethylamine or water is used as a reaction solvent; however, the method (7) has problems in that the selectivity for cyclohexanone oxime is as low as about 30 to 50% and also the catalyst activity is low.
Nitrone compounds, which are induced by oxidation of amines, are important chemical compounds for use as intermediates for synthesis of pharmaceuticals, agrichemicals and fine chemicals, such as an α-substituted amine compound, an amino acid and an alkaloid. As a method for producing a nitrone compound, there is known a method in which a secondary amine is reacted with hydrogen peroxide. Examples of such methods for producing a nitrone compound include a method which uses a sodium tungsten catalyst (see Unexamined Japanese Patent Application Laid-Open Specification No. Sho 59-164762 (corresponding to U.S. Pat. No. 4,596,874)), a method which uses a selenium dioxide catalyst (see Unexamined Japanese Patent Application Laid-Open Specification No. Sho 63-63651), and a method which uses a methyltrioxorhenium catalyst (see Bull. Chem. Soc. Jpn., 70, 877, 1997). However, these methods have problems not only in that hydrogen peroxide (which is expensive) or an expensive catalyst is used, but also in that the catalyst activity is unsatisfactory and the selectivity for and yield of a nitrone compound are not always high. Thus, these methods are unsatisfactory from the commercial viewpoint. There is also known another method for producing a nitrone compound, which uses a molecular oxygen as an oxidizing agent and uses hydrazine (reducing agent) and a flavin catalyst (see J. Am. Chem. Soc., 125, 2868, 2003). This method exhibits a high selectivity for a desired chemical compound; however, this method has problems not only in that hydrazine as a reducing agent is expensive, but also in that the catalyst has disadvantages in that the preparation of the catalyst requires a complicated operation and, hence, the catalyst is expensive. Thus, this method is unsatisfactory from the commercial viewpoint.
As apparent from the above, it has been desired to develop an oxidation method which can be used to produce various desired chemical compounds (for example, to produce an oxime compound or a nitro compound from a primary amine or produce a nitrone compound from a secondary amine) by an oxidation reaction (such as an oxygen oxidation reaction) performed under moderate conditions with high selectivity and high efficiency. It has also been desired to develop a high performance oxidation catalyst used for the oxidation method.